1. Field of the Invention
The invention pertains to the field of addition polymerization. More particularly, the invention pertains to methods of controlling the molecular weight of a product of addition polymerization.
2. Description of Related Art
Addition polymerization, also known as polyaddition and chain growth polymerization, occurs by stepwise addition of monomer units onto a growing polymer chain. Addition polymerization occurs in three distinct steps. Each chain is first initiated, then propagated, and finally terminated. Since polymerization occurs by stepwise addition of monomer units, the average molecular weight of the polymer chains increases linearly with reaction time for a given set of initial reaction conditions, assuming excess monomer is present.
Polymers that have been formed by addition polymerization include, but are not limited to, polyolefins, polycarbonates, polyesters, and polyethers. Examples of addition polymers include, but are not limited to, polyethylene (PE), polypropylene (PP), poly(vinyl chloride) (PVC), polystyrene (PS), polyacrylonitrile, poly(vinyl acetate) (PVA), poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVA), polytetrafluoroethylene (PTFE), poly(cyclohexene carbonate) (PCHC), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), and poly(propylene oxide) (PPO).
Coates and Moore (Angew. Chem., Int. Ed., 43, pp. 6618-6639, 2004, hereby incorporated herein by reference) review catalysts for co-polymerization of carbon dioxide and epoxides by addition polymerization. These metal-based catalysts include, but are not limited to, aluminum-, manganese-, chromium-, cobalt-, yttrium-, zinc-, cadmium-, and nickel-based catalysts. Qin et al. (Angew. Chem., Int. Ed., 42, pp. 5484-5487, 2003), Cohen et al. (J. Am. Chem. Soc., 127, pp. 10869-10878, 2005), and U.S. Pat. No. 7,304,172, issued to Coates et al., which are all hereby incorporated herein by reference, disclose cobalt catalysts for co-polymerization of alkylene oxides and carbon dioxide. Coates et al. (Angew. Chem. Int. Ed., 43, pp. 6618-6639, 2004) and U.S. Pat. No. 6,133,402 issued to Coates et al. which are hereby incorporated herein by reference disclose zinc-based and other metal-based catalysts for the co-polymerization of alkylene oxides and carbon dioxide.
One or more co-catalysts may be used in conjunction with a catalyst in an addition polymerization to increase the rate of polymerization and potentially allow for a decrease in pressure or temperature for the reaction in comparison to use of the catalyst alone. A co-catalyst may also improve the stereoselectivity or regioselectivity of a polymerization reaction. Coates et al. (U.S. Pat. No. 7,304,172) disclose bis(triphenylphosphine)iminium chloride, bis(triphenylphosphine)iminium pentafluorobenzoate, tetraphenylphosphonium bromide, tetraphenylphosphonium chloride, tetra-n-butylammonium chloride, triethylamine, and trioctylamine as co-catalysts for use with cobalt catalysts for co-polymerization of alkylene oxide and carbon dioxide.
A reversible chain transfer agent may be used in conjunction with a catalyst to decrease the average molecular weight of the product without otherwise significantly affecting the addition polymerization reaction. Coates et al. (U.S. Pat. No. 7,304,172) disclose carboxylic acids, such as pentafluorobenzoic acid, alcohols, such as methanol, dicarboxylic acids, diols, poly acids, polyols, and their deprotonated forms, such as sodium pentafluorobenzoate, as reversible chain transfer agents for use with cobalt catalysts for co-polymerization of propylene oxide and carbon dioxide.